This invention relates to the field of detectors for ionising alpha particle radiation from airborne radioactive nuclei that use ambient air as the detection medium.
The US National Academy of Sciences report BEIR VI (released 19th Feb. 1998) indicates that the pollution of buildings by radon (a naturally occurring radioactive noble gas) causes about 15,000 lung cancer deaths per annum in America and corresponding mortality elsewhere. Radon levels are not predictable: adjacent buildings of similar construction often have very different interior levels of radon and high levels of radon are found over various bed-rocks. The American Environment Protection Agency (EPA) recommends that every dwelling be checked for radon. Fortunately if radon pollution is detected it can be reduced by simple and inexpensive techniques. Continuous measurement of radon is desirable because levels change due to tectonic plate movement, seasonal heating and ventilation changes and building modifications, and to validate protection methods. Additionally radon levels often change before earthquakes. Monitoring radon levels in real time across a large area, for example around known fault lines, and communicating these observations over a network to responsible persons may be used to warn of an imminent earthquake. Because the radiation exposure may vary with time so that e.g. the total exposure over say 24 hours may consist of several hours of high levels of airborne alpha particle radiation with lower levels for the other hours, it is desirable to sample the level of radiation over a plurality of time periods. Other potential sources of airborne alpha particle radiation include plutonium and uranium. Such elements may be put into the atmosphere by mining, accidents at a nuclear reactor or nuclear fuel or weapons processing facility or with a nuclear weapon or weapons, or military conflict with nuclear weapons and or depleted uranium munitions, or military conflict or terrorist action in which sites containing radioactive material are attacked, or by natural disasters such as earthquakes or volcanoes. Following such releases alpha particle radioactive material may remain in the air for considerable periods, or be deposited on the ground and then returned to the atmosphere by winds, agricultural activity, vehicle movement and such. It is desirable to have detectors that provide real time warning of airborne alpha particle radiation. If such airborne radiation is detected, simple actions such as changing building ventilation and/or using simple filters, such as handkerchiefs, over the mouth can reduce exposure and risk of illness. Such detectors may also be networked together to provide an overall picture of the levels of airborne alpha particle radiation.
Two classes of practical alpha particle detectors exist. (1) Low cost devices with no real time monitoring, that are deployed for extended periods. Usually only the integrated radon exposure is determined, often after laboratory processing. (2) Sophisticated and expensive detectors that require skilled operators and which provide real time data. Devices based around ambient air ion counters exist in both classes. Low cost ion counters detect the rate of decay of a charged electrode, such as an electret, caused by ionising radiation. Sophisticated ion counters may detect an integrated direct current caused by atmospheric ionising radiation, or count individual alpha particle decays either with sophisticated low-current measuring apparatus such as KEITHLEY™ (of Ohio, USA) electrometers or with sophisticated digital signal processing (DSP) techniques. Because of the inherent slow charge collection of atmospheric ion counters conventional fast signal shaping can not be used and such detectors are often troubled by microphonics necessitating careful disturbance free siting and/or sophisticated DSP techniques to prevent erroneous measurement. Such requirements are incompatible with the robustness, maintenance free operation and low cost essential for a mass-market detector. All of the prior art has disadvantageous features including expense and sophistication necessitating skilled operation, the long time delay between measurement and results, the absence of long term real-time measurement, the size and bulk of the detectors, and the sensitivity to microphonics. Such disadvantages have prevented the large-scale use of continuous readout radon detectors. There has not previously been a proposal for a truly practical detector for airborne alpha particle radiation that can be sized suitably for domestic applications and which provides continuous measurement over a plurality of time periods, requires minimum maintenance and is easily manufactured from commonly available components, and which does not require a specially trained person to operate.